Place the xpath restriction on the account entity in the domain model. [id = $currentUser]
in the nanoflow just retrieve the first account and it will be constrained by the xpath.
Probably you can use Xpath constraint with '[%CurrentUser%]'] restriction directly on entity. User is system entity, and you can’t modify access rules, but option is use associated/generalized entity (like account). After that when you try to retrive all account from your db, you get list with only one element.
Hope this will help